CN114492234A - Dynamic floating type fan surface rainwater distribution and rainwater load calculation method - Google Patents

Dynamic floating type fan surface rainwater distribution and rainwater load calculation method Download PDF

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CN114492234A
CN114492234A CN202210029516.3A CN202210029516A CN114492234A CN 114492234 A CN114492234 A CN 114492234A CN 202210029516 A CN202210029516 A CN 202210029516A CN 114492234 A CN114492234 A CN 114492234A
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孙寒冰
吴松
郑兴
郝红彬
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Abstract

The invention belongs to the technical field of computational fluid mechanics, and particularly relates to a method for calculating rainwater distribution and rainwater load on the surface of a dynamic floating type fan. The method comprises the steps of measuring the wind speed, the wind pressure and the wind direction of a wind field around a dynamic floating type fan, and constructing a wind field calculation domain in a stable state by taking measured data as boundary conditions; measuring the rainfall intensity of the offshore area of the working position of the dynamic floating type fan, and injecting a rain phase into a numerical wind field; constructing a wind and rain field SRF calculation system, and solving a wind and rain coupling effect control equation; calculating to obtain surface rainwater distribution based on rainfall event observation data; and calculating the impact force of the rainwater on the surface infinitesimal area of the fan on the collision with the fan, and further integrating the whole surface of the fan to obtain the surface rain load of the fan. The method can be used for calculating the rain load of a static building and calculating the rain load of a fan with dynamic characteristics.

Description

Dynamic floating type fan surface rainwater distribution and rainwater load calculation method
Technical Field
The invention belongs to the technical field of computational fluid mechanics, and particularly relates to a method for calculating rainwater distribution and rainwater load on the surface of a dynamic floating type fan.
Background
War (wind And rain) refers to the phenomenon that raindrops that originally fall vertically under the influence of wind have a horizontal velocity vector. WAR is one of the important sources of moisture that affects the wet heat performance and durability of exterior surfaces of machinery, buildings, etc., and not only can generate additional force on the acted object to affect the performance of the acted object, but also can cause some undesirable phenomena in the surface physics, such as frostbite of the exterior wall surface and erosion of materials. For the floating type fan, due to the particularity of the environment, the floating type fan works under the working condition of combined action of wind and rain, so that the WAR phenomenon has obvious influence on mechanical abrasion, working efficiency and the like of the fan, and the WAR facade distribution has important promotion effects on waterproof design, waterproof material development, warm and humid performance research and the like of an object building, so that the determination of the surface rainwater distribution and the rain load characteristic of the fan in a rotating state has obvious significance for the floating type fan.
In response to this problem, the inventors have found the following disadvantages based on the prior art method:
at present, there are three methods to estimate the WAR rain distribution and load effect on the surface of an object building: (1) measurement, (2) semi-empirical method, (3) Computational Fluid Dynamics (CFD) method.
Firstly, the measurement of the object facade WAR is difficult, time-consuming, and prone to error, and they are also limited by the meteorological conditions during the experiment.
Second, semi-empirical methods are fast and easy to use, but they only give an approximation of the WAR strength and do not provide detailed information. Furthermore, semi-empirical methods cannot reliably account for all factors that affect the strength of the WAR, particularly in multi-building environments.
Thirdly, the current CFD method adopts the euler two-phase flow-based lagrangian particle tracking method (LPT model) for the calculation of the rainfall condition, and the model needs to track thousands of raindrops on a fine calculation grid to obtain an accurate result, so that the calculation cost is too much for many researchers to expect. Researchers must carefully define the raindrop injection location for each raindrop diameter so that they cover the entire facade. This step must be repeated for different values of reference wind speed and reference wind direction. Furthermore, this method requires calculations to be performed in very small time steps to obtain accurate results. Therefore, all steps of LPT, i.e., pre-processing, solving, and post-processing, are very time consuming. This is mainly true in situations where the researcher is time-intensive and has sufficient computing resources. Furthermore, the research of the CFD method under the wind and rain working conditions is only focused on static buildings.
Therefore, for a fan with dynamic characteristics under the wind and rain working conditions, an accurate, efficient and wide-applicability rain load calculation method is awaited.
Disclosure of Invention
The invention aims to provide a method for calculating the rainwater distribution and the rainwater load on the surface of a dynamic floating type fan.
A method for calculating the rainwater distribution and the rainwater load on the surface of a dynamic floating type fan is characterized by comprising the following steps:
step 1: measuring the wind speed, wind pressure and wind direction of a wind field around the dynamic floating type fan, and constructing a stable wind field calculation domain by using the measured data as boundary conditions;
the control equation for the wind field calculation domain at steady state is as follows:
Figure BDA0003465717960000021
Figure BDA0003465717960000022
Figure BDA0003465717960000023
Figure BDA0003465717960000024
Figure BDA0003465717960000025
wherein x isi、xjRespectively representing the displacement in the ith and the j directions; u. ofi、ujRespectively representing the average wind speed in the ith and the jth directions; rhoaRepresents the density of air; p is the pressure of air; k represents turbulent kinetic energy; ε represents the turbulent dissipation ratio; tau isijRepresents the reynolds stress; μ represents air viscosity; mu.stRepresents the air turbulence viscosity; gKRepresenting the turbulent kinetic energy gradient produced by the average velocity; cμ、C2ε、C1εAnd σεAre all constants;
step 2: measuring rainfall R of offshore area of dynamic floating type fan working positionh(ii) a Determining the rainfall intensity R on the basis of the known rainfall intensityhThe rains are composed of raindrops of different sizes and the falling speeds of the raindrops of different sizes; adding different continuous rain phases consisting of raindrops with different sizes on the top and the inlet of a wind field calculation domain in a stable state to construct an Euler multiphase field, and taking the measured raindrop size, raindrop falling speed and rain phase fraction as boundary conditions of the Euler multiphase field;
the raindrop falling process can be regarded as a process of accelerating firstly and then keeping constant speed, and the raindrop can keep a constant final speed before reaching the ground, so that the initial speed of the raindrop phase on the boundary condition of the calculation domain is the final speed V of the raindropk(dk)，dkDiameter of raindrop of kth-phase rain, Vk(dk) Denotes the diameter dkThe end-of-raindrop velocity of the kth-phase rain; rain phase fraction alphakThe calculation method comprises the following steps:
Figure BDA0003465717960000026
setting the boundary conditions of the rain phases of the surface of the fan, the ground and the outlet as follows: rain phase fraction gradient when normal wind speed velocity vector is indicated from the calculation field
Figure BDA0003465717960000027
Equal to zero, rain phase fraction alpha when normal wind speed velocity vector points to the calculation domainkIs equal to zero; by utilizing the boundary condition, the interaction between rain and the outer wall and the surface of the fan can be not considered, and once the raindrops reach the boundary of the wall, the raindrops leave the area, so that the energy loss of other factors can be ignored;
and step 3: simulating the process from starting the dynamic floating fan in the weather to maintaining the stable state, and adopting a single rotating reference system method, namely, the fan is not moved, a coordinate system rotates and coordinate transformation is carried out on the amount in the coordinate system, so as to solve the calculation domain;
combining the rain phase and the wind phase, and adopting the assumption of unidirectional coupling in calculation, namely that the wind acts on rain in a unidirectional way; the rain phase is considered as a continuum, each rain corresponds to a different level of raindrop size; for each rain phase, after the rain phase is injected into wind, solving the following continuity and momentum equation when the rain phase is in single-term coupling with the wind phase to obtain rain phase fraction and speed field information;
Figure BDA0003465717960000031
Figure BDA0003465717960000032
wherein alpha isk' calculating the rain phase fraction of the kth phase rain after the wind phase is coupled with the rain phase in the domain; rhowIs raindrop density; g is the acceleration of gravity; cdIs a coefficient of resistance; reRThe relative reynolds number is the relative reynolds number,
Figure BDA0003465717960000033
Figure BDA0003465717960000034
is the vector of the velocity of the wind phase,
Figure BDA0003465717960000035
is the rain phase velocity vector;
and 4, step 4: calculating to obtain surface rainwater distribution based on rainfall event observation data;
the parameters defining the distribution of rain on the external surface of the building under the action of wind and rain are capture ratio, which is defined as the ratio of the intensity of rain under the action of wind to the intensity of rain on the horizontal plane, and the size of the global capture ratio eta is equal to the capture ratio specific to each rain phase
Figure BDA0003465717960000036
The two are directly related, and the calculation formulas are as follows:
Figure BDA0003465717960000037
Figure BDA0003465717960000038
wherein u (k) is the final collision velocity vector finally calculated by the k-th phase rain; f. ofh(dk) As intensity of rainfall RhLower, the k-th phase diameter is dkThe size probability distribution value of the rain phase on the horizontal plane;
and 5: calculating the surface infinitesimal area delta of the fansThe rainwater impacts the impact force F of the fan, so that the integral surface of the whole fan is integrated to obtain the surface rain load of the fan;
fan surface infinitesimal area deltasThe impact force of the upward rainwater collision fan is as follows:
F＝ρwηRhΔsu
wherein u is the resultant velocity of the final velocities of all the rain phases before impacting the wall surface of the fan.
The invention has the beneficial effects that:
the invention couples an Euler multiphase model with a multiple rotation reference System (SRF) and a collision theory, and is used for calculating the rainwater distribution on the surface of the dynamic rotation floating type fan and the rainwater load. The method can be used for calculating the rain load of the static building and the rain load of the fan with the dynamic characteristic, and solves the problems that the existing calculation method is long in time consumption, complex in operation, high in calculation resource consumption, capable of calculating only the static building and the like in the process of calculating the wind and rain load.
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Fig. 1 is a schematic diagram of the present invention.
Fig. 2 is a data graph of raindrop size distribution through a horizontal plane calculated by Best measurement.
FIG. 3 is a graph of end velocity size distributions of raindrops of different sizes measured by Gunn and Kinzer.
Fig. 4 is a schematic diagram of a rotating reference frame (SRF).
FIG. 5 is a diagram of a calculation result of the distribution of rainwater on the surface of the fan.
FIG. 6 is a graph of the results of a fan blade rain induced pressure distribution.
Fig. 7 is a display diagram of the wind phase and rain phase streamlines after the calculation is completed.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
In order to solve the problems that the existing calculation method has many limitations in calculating wind and rain loads, such as long time consumption, complex operation, high calculation resource consumption, calculation of static buildings only and the like, the invention provides a method for calculating the distribution of rainwater on the surface of a dynamic rotating floating type fan and the load of the rainwater on the surface of the dynamic rotating floating type fan by coupling an Euler multiphase model with a multiple rotating reference System (SRF) and a collision theory based on computational fluid mechanics.
With reference to fig. 1, taking an example of a floating wind turbine installed in a certain offshore area for operation, a method for calculating surface rainwater distribution and rainwater load of a dynamic floating wind turbine specifically includes the following steps:
step one, measuring required wind field data and constructing a wind field;
firstly, relevant data (wind speed, wind pressure and wind direction) of a wind field at a boundary of 200 meters around a floating type fan are measured, and the measured wind field data are used as boundary conditions of a constructed numerical wind field. The interaction process of the incompressible turbulent wind and the fan is calculated by adopting stable Reynolds average Navier-Stokes (RANS) and KOmegaSST turbulence models, and the information of the flow field after the interaction between the incompressible turbulent wind and the fan is stable can be calculated. The KOmegaSST turbulence model considers the transmission of turbulence shear stress on the basis of a standard k-omega model, and also considers low Reynolds number, compressibility and shear flow propagation. The control equation is as follows:
Figure BDA0003465717960000041
Figure BDA0003465717960000042
Figure BDA0003465717960000043
Figure BDA0003465717960000044
Figure BDA0003465717960000051
wherein x isi、xjRespectively representing the displacement in the ith and the j directions; u. ofi、ujRespectively representing the average wind speed in the ith and the jth directions; rhoaRepresents the density of air; p is the pressure of air; k represents turbulent kinetic energy; ε represents the turbulent dissipation ratio; tau isijRepresents the reynolds stress; μ represents air viscosity; mu.stRepresents the air turbulence viscosity; gKRepresenting the turbulent kinetic energy gradient produced by the average velocity; cμ、C2ε、C1εAnd σεAre all constants; respectively taking Cμ＝0.11、C2ε＝1.92、C1ε1.44 and σε＝1.21。
Measuring rainfall intensity, and injecting a rain phase into a numerical wind field;
measuring the descent of an offshore region of a dynamic floating wind turbineRain intensity RhOn the basis of the known rainfall intensity, the rain at the rainfall intensity is determined to be composed of raindrops with different sizes and the falling speed of the raindrops with different sizes. Injecting different continuous rain phases consisting of raindrops with different sizes into the top of the stable wind field calculation domain obtained after calculation to construct an Euler multiphase field, and taking the measured rainfall intensity related parameters (raindrop size, raindrop falling speed and rain phase fraction) as boundary conditions of the Euler multiphase field.
Different rain phases (one rain phase represents rain drops with one size) are added to the top and the inlet of a stable wind field watershed, fixed rainfall intensity is selected for research, a set of rain drop combinations with different sizes are correspondingly selected for each fixed rainfall intensity, for example, the rain intensity of 0.1mm/h is selected, twenty-phase rain is set up at present, each rain phase has a phase fraction and a velocity vector, the rain drop diameters of the twenty-phase rain phases range from 0.1 to 1mm at intervals of 0.1mm, 1 to 2mm at intervals of 0.2mm, 2 to 7 mm at intervals of 1mm, and the rain drop diameters are still selected according to a data graph of rain drop size distribution calculated by Best measurement and passing through a horizontal plane, and the data graph is shown in figure 2.
The boundary condition of the rain phase is still determined by the end velocity V of the raindropsk(dk) Phase fraction alphakTwo-parameter control, the solution of these two parameters on the boundary conditions will be described below. The raindrop falling process can be regarded as a process of accelerating first and then keeping constant speed, and the raindrop can keep a constant final speed before reaching the ground, so that the initial speed of the raindrop phase on the boundary condition of the calculation domain is the final speed V of the raindropk(dk). End raindrop velocity sizing was performed by Gunn and Kinzer as shown in fig. 3.
The size of the rain phase fraction on the boundary condition is also a fixed value, and is calculated by formula 6. dkDiameter of raindrop of kth-phase rain, Vk(dk) Denotes the diameter dkThe end-of-raindrop velocity of the k-th phase rain.
Figure BDA0003465717960000052
For the setting of the boundary conditions of the rain phases of the surface of the fan, the ground and the outlet, the following self-programming boundary conditions are adopted: the phase fraction gradient equals zero, and the phase fraction gradient of rain when the normal wind velocity vector points out of the calculation field
Figure BDA0003465717960000053
Equal to zero, phase fraction alpha when the normal wind speed velocity vector points to the computation domaindIs equal to zero. With this boundary condition, the interaction between rain and the wall (outer wall, fan surface) can be disregarded, and the raindrops leave the area once they reach the wall boundary, so the energy loss of other factors, such as evaporation, splashing, breaking of raindrops during impact, etc., can be neglected.
Thirdly, applying a rotating reference system model;
after the calculation of the steady-state wind field is finished, a rain phase is injected into the calculation domain, the fan starts to rotate, the process from starting of the 5MW fan in the wind and rain to maintaining the steady state is simulated, and because the research only focuses on the relevant aerodynamic performance of the fan after the fan rotates in the wind and rain to maintain the steady state, a common SRF method (single rotating reference system method) for CFD (continuous form-factor calibration) is adopted, namely the fan is not moved, the coordinate system rotates, the coordinate transformation is carried out on the quantity in the coordinate system, and then the calculation domain is solved. The SRF method is different from a sliding grid, and grid movement and exchange do not exist, so that the time cost of calculation is greatly saved. Fig. 4 is a schematic diagram of SRF distinguished from a slipping grid.
Solving a wind-rain coupling effect control equation;
the rain phase and the wind phase are combined, and the calculation adopts the assumption of unidirectional coupling, namely that the wind acts on the rain in a unidirectional mode. This is a valid assumption, since the volume ratio of rain in air is less than 1 x 10-4. The rain phase is considered to be a continuum, each rain corresponds to the size of raindrops of different levels, and for each rain phase, after the rain phase is injected into wind, the following continuity and momentum equations in the process of univariate coupling with the wind phase are solved, so that flow field information such as rain phase volume fraction (phase fraction) and velocity field can be obtained. FIG. 7 shows a flow fieldFlow charts of the medium rain phase and the wind phase.
Figure BDA0003465717960000061
Figure BDA0003465717960000062
Wherein alpha isk' calculating the rain phase fraction of the kth phase rain after the wind phase is coupled with the rain phase in the domain; rhowIs raindrop density; g is the acceleration of gravity; cdIs a coefficient of resistance; reRThe calculation formula is as follows for relative Reynolds number:
Figure BDA0003465717960000063
wherein,
Figure BDA0003465717960000064
is the vector of the velocity of the wind phase,
Figure BDA0003465717960000065
is the rain phase velocity vector;
calculating to obtain surface rainwater distribution based on rainfall event observation data;
and after the rain phase motion trail and the control parameters are obtained, the distribution of the rainwater on the outer surface of the building is obtained based on raindrop distribution probability graphs of different sizes measured by Best.
The parameter defining the distribution of rain on the outer surface of the building under the action of wind and rain is the capture ratio, which is defined as the ratio of the intensity of rain under the action of wind to the intensity of rain on the horizontal plane (i.e. the normal rain intensity). The size of the global capture ratio eta is equal to the capture ratio specific to each rain phase
Figure BDA0003465717960000066
The two are directly related, and the calculation formulas are as follows:
Figure BDA0003465717960000067
Figure BDA0003465717960000071
wherein, Rwdr(k) The intensity of the action of wind on rain; u (k) final calculated end-of-impact velocity vector for kth phase rain; f. ofh(dk) As intensity of rainfall RhLower, the k-th phase diameter is dkThe size probability distribution value of the rain phase on the horizontal plane, and particularly, refer to fig. 2. FIG. 5 shows the final calculation result of the distribution of the rain on the surface of the fan at the rainfall intensity of 5 mm/h.
And step five, calculating to obtain the rain load by using a collision theory.
The interaction process between rain and structures follows newton's second law.
By the formula of the theorem of momentum:
Figure BDA0003465717960000072
wherein tau is the time of rain impacting the wall, f (t) is the rain impact force vector, u is the sum of the final speeds of all rain phases before impacting the wall of the fan, and m is the rain phase quality.
So the impact force is:
Figure BDA0003465717960000073
because the diameter of the raindrop is generally small and the final speed before the raindrop impacts the wall surface is large, the collision time is taken for simplifying the calculation
Figure BDA0003465717960000074
And because the definition of the surface capture ratio of the fan is as follows: the ratio of the rain intensity under the action of wind to the rain intensity of a horizontal plane, namely the rain intensity of the surface of the fan is as follows:
R＝η×Rh (15)
wherein eta is the surface capture ratio of the fan, RhIs the horizontal rainfall intensity. Whereas the rain intensity is equal to the volume of rainfall over a certain area per unit time. So that the surface infinitesimal area delta of the fan in the time tausThe quality of the upper rainwater is as follows:
m＝ρwηRhΔsτ (16)
fan surface infinitesimal area deltasThe impact force of the upward rainwater collision fan is as follows:
F＝ρwηRhΔsu (17)
and (4) integrating the whole surface of the fan, and calculating the surface rain load of the whole fan.
Fig. 6 illustrates the rain induced pressure distribution of the fan blades A, B, C along the radial direction. The selected working conditions of the offshore area are 6m/s and 11.4m/s of horizontal wind speed, 50N of wind pressure, 12.1RPM of rotating speed and 5mm/h of rain strength.
The method can be used for calculating the rain load of a static building and the rain load of the fan with dynamic characteristics, and solves the limitations and the blanks of fan rain distribution and rain load calculation under the wind and rain working conditions at home and abroad at present.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (1)

1. A method for calculating the rainwater distribution and the rainwater load on the surface of a dynamic floating type fan is characterized by comprising the following steps:
step 1: measuring the wind speed, wind pressure and wind direction of a wind field around the dynamic floating type fan, and constructing a stable wind field calculation domain by using the measured data as boundary conditions;
the control equation for the wind field calculation domain at steady state is as follows:
Figure FDA0003465717950000011
Figure FDA0003465717950000012
Figure FDA0003465717950000013
Figure FDA0003465717950000014
Figure FDA0003465717950000015
wherein x isi、xjRespectively representing the displacement in the ith and the j directions; u. ofi、ujRespectively representing the average wind speed in the ith and the jth directions; rhoaRepresents the density of air; p is the pressure of air; k represents turbulent kinetic energy; ε represents the turbulent dissipation ratio; tau isijRepresents the reynolds stress; μ represents air viscosity; mu.stRepresents the air turbulence viscosity; gKRepresenting the turbulent kinetic energy gradient produced by the average velocity; cμ、C2ε、C1εAnd σεAre all constants;
step 2: measuring rainfall R of offshore area of dynamic floating type fan working positionh(ii) a Determining the rainfall intensity R on the basis of the known rainfall intensityhThe rains are composed of raindrops of different sizes and the falling speeds of the raindrops of different sizes; adding different continuous rain phases composed of raindrops with different sizes on the top and the inlet of a wind field calculation domain in a stable state to construct an Euler multiphase field, and measuring the sizes and the raindrops of the raindropsThe landing speed and the rain phase fraction are used as boundary conditions of the Euler multiphase field;
the raindrop falling process can be regarded as a process of accelerating first and then keeping constant speed, and the raindrop can keep a constant final speed before reaching the ground, so that the initial speed of the raindrop phase on the boundary condition of the calculation domain is the final speed V of the raindropk(dk)，dkDiameter of raindrop of kth-phase rain, Vk(dk) Denotes the diameter dkThe end-of-raindrop velocity of the kth-phase rain; rain phase fraction alphakThe calculation method comprises the following steps:
Figure FDA0003465717950000016
setting the boundary conditions of the rain phases of the surface of the fan, the ground and the outlet as follows: rain phase fraction gradient when normal wind speed velocity vector is indicated from the calculation field
Figure FDA0003465717950000017
Equal to zero, rain phase fraction alpha when normal wind speed velocity vector points to the calculation domainkIs equal to zero; by utilizing the boundary condition, the interaction between rain and the outer wall and the surface of the fan can be not considered, and once the raindrops reach the boundary of the wall, the raindrops leave the area, so that the energy loss of other factors can be ignored;
and step 3: simulating the process from starting the dynamic floating fan in the weather to maintaining the stable state, and adopting a single rotating reference system method, namely, the fan is not moved, a coordinate system rotates and coordinate transformation is carried out on the amount in the coordinate system, so as to solve the calculation domain;
combining the rain phase and the wind phase, and adopting the assumption of unidirectional coupling in calculation, namely that the wind acts on rain in a unidirectional way; the rain phase is considered as a continuum, each rain corresponds to a different level of raindrop size; for each rain phase, after the rain phase is injected into wind, solving the following continuity and momentum equation when the rain phase is in single-term coupling with the wind phase to obtain rain phase fraction and speed field information;
Figure FDA0003465717950000021
Figure FDA0003465717950000022
wherein alpha isk' calculating the rain phase fraction of the kth phase rain after the wind phase is coupled with the rain phase in the domain; rhowIs raindrop density; g is the acceleration of gravity; cdIs a coefficient of resistance; reRThe relative reynolds number is the relative reynolds number,
Figure FDA0003465717950000023
Figure FDA0003465717950000024
is the vector of the velocity of the wind phase,
Figure FDA0003465717950000025
is the rain phase velocity vector;
and 4, step 4: calculating to obtain surface rainwater distribution based on rainfall event observation data;
the parameters defining the distribution of rain on the external surface of the building under the action of wind and rain are capture ratios, which are defined as the ratio of the intensity of rain under the action of wind to the intensity of rain on the horizontal plane, and the size of the global capture ratio eta is equal to the capture ratio eta specific to each rain phasedk(k) The two are directly related, and the calculation formulas are as follows:
Figure FDA0003465717950000026
Figure FDA0003465717950000027
wherein u (k) is the final collision velocity vector finally calculated by the k-th phase rain; f. ofh(dk) As intensity of rainfall RhLower, the k-th phase diameter is dkThe size probability distribution value of the rain phase on the horizontal plane;
and 5: calculating the surface infinitesimal area delta of the fansThe upper rainwater collides with the impact force F of the fan, and then the integral of the whole surface of the fan is obtained to obtain the surface rain load of the fan;
fan surface infinitesimal area deltasThe impact force of the upward rainwater collision fan is as follows:
F＝ρwηRhΔsu
wherein u is the resultant velocity of the final velocities of all the rain phases before impacting the wall surface of the fan.
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